Various optical spectroscopic measurement systems have been developed for the noninvasive monitoring of blood constituent concentrations. In such systems, light of multiple wavelengths is used to illuminate a thin tissue portion of a person, such as a fingertip or earlobe, to obtain a spectrum analysis of the light absorbed by blood flowing through the tissue site. Pulse oximetry systems, which perform such measurements to monitor blood oxygenation of hemoglobin constituents, have been particularly successful in becoming the standard of care. Extending this technology to the noninvasive monitoring of other blood constituents, such as blood glucose, is highly desirable. For example, current methods for accurately measuring blood glucose involve drawing blood from the subject, which can be onerous for diabetics who must take frequent samples to closely monitor blood glucose levels.
FIG. 1 illustrates an optical spectroscopic measurement system 100. A multiple wavelength light source 110 produces incident light 112 of intensity I0 and wavelength λ, I0,λ, which illuminates a sample 120 having multiple constituents, each of concentration ci. The incident light 112 is partially absorbed by the sample 120, and transmitted light 130 of intensity I emerges from the sample 120. A detector 140 provides an output signal 142 that is proportional to the transmitted light 130. A signal processor 150 operates on the detector output signal 142 to provide a measurement 152 that is indicative of one or more of the constituent concentrations ci in the sample 120, based upon the known extinction coefficients εi,λ of the sample constituents.